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MP4021A

Primary Side Control Offline LED Controller with Active PFC

Monolithic Power Systems 

MP4021A—PRIMARY SIDE CONTROL OFFLINE LED CONTROLLER WITH ACTIVE PFC

Power Factor Correction

The MULT pin is connected to the tap of the

resistor divider from the rectified instantaneous

line voltage and fed as one input of the Multiplier.

The output of the multiplier will be shaped as

sinusoid too. This signal provides the reference

for the current comparator and comparing with

the primary side inductor current which sets the

primary peak current shaped as sinusoid with the

input line voltage. High power factor can be

achieved.

Multiplier output

Inductor current

off, if ZCD fails to send out another turn on signal

after 130µs, the starter will automatically send

out the turn on signal which can avoid the IC

unnecessary shut down by ZCD missing

detection.

Minimum Off Time

The MP4021A operates with variable switching

frequency, the frequency is changing with the

input instantaneous line voltage. To limit the

maximum frequency and get a good EMI

performance, MP4021A employs an internal

minimum off time limiter—3.5µs, show as Figure

6.

ZCD

Figure 4—Power Factor Correction Scheme

The maximum voltage of the multiplier output to

the current comparator is clamped to 2.5V to get

a cycle-by-cycle current limitation.

VCC Under-voltage Lockout

When the VCC voltage drops below UVLO

threshold 9V, the MP4021A stops switching and

totally shuts down, the VCC will restart charging

by the external start up resistor from AC line.

Figure 5 shows the typical waveform of VCC

under-voltage lockout

Vcc

Auxiliary Winding Takes Charge Protection happens

And Regulates the VCC

13.6V

9V

Gate

Switching Pulses

GATE

3.5us

Figure 6—Minimum Off Time

Leading Edge Blanking

In order to avoid the premature termination of the

switching pulse due to the parasitic capacitance

discharging at MOSFET turning on, an internal

leading edge blanking (LEB) unit is employed

between the CS Pin and the current comparator

input. During the blanking time, the path, CS Pin

to the current comparator input, is blocked.

Figure 7 shows the leading edge blanking.

VCS

tLEB =280 ns

Figure 5—VCC Under-Voltage Lockout

Auto Starter

The MP4021A integrates an auto starter, the

starter starts timing when the MOSFET is turned

t

Figure 7—Leading Edge Blanking

MP4021A Rev.1.04

www.MonolithicPower.com

13

7/5/2013

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